Ford 460 Stroker Piston Selection

Question: Just to start things off, I'm 20, and this is my first engine build, so if I leave out calculations or other things, that's why!

I'm building up a '78 Ford 460 block. I've bought a set of D0VE-C heads for it, and I'm planning on getting a 4.500-inch stroker crank. The rod length I'm supposed to use with the crank is 6.800. I will bore the engine 0.030-over to leave some room for a future rebuild. I plan on installing a decently radical cam as well, something between 0.400-0.500 lift. As for the heads, I'm going to have a port-and-polish as well as a valve job done.

Taking note of all the above, how do I go about piston selection? I'm definitely going with forged, so I have room to upgrade (nitrous, forced induction, and so on). I'm basically worried about not having enough clearance in the cylinder. With the smaller combustion chamber on the D0VE heads versus the stockers, as well as the stroke being longer and a bigger cam, how do I figure out what piston I need? I'm hoping to get the compression up to about 12.5:1.Daryl GreenleeMeadville, PA

1/4At TDC, the piston must physically fit within the constraints of the block height and the remainder of the rotating assembly. Piston height is determined by subtracting the rod length and half the stroke from the engine's blueprint block height.

Answer: Don't be afraid to ask questions-we all had to start somewhere. That said, engine math is an important part of hot-rod engine building, so here's what you (or your custom aftermarket piston maker) need to consider.

First, determine if it's even possible to make a piston that fits. Is there enough physical space in the cylinder block to accommodate the piston, its piston rings, and the piston pin with your crank stroke and connecting-rod length? Start by figuring out the max possible piston height-known as compression distance (CD)-that's available to work with. CD is the distance from the pin centerline to the piston deck (the flat part of the piston top). To find CD, subtract the rod length and half the stroke from the engine's blueprint block height. The design blueprint block height of a '72-'96 Ford 460 block is 10.322 inches from the crank centerline to the block-deck surface (block-milling will probably be needed to achieve the blueprint dimension).

So you have 1.272 inches in which to accommodate the piston CD and still achieve at least a zero piston deck clearance at TDC.

Second, you need to figure if the calculated max CD is sufficient to accommodate the ring package. According to Hank Bechtloff, the legendary Hank The Crank, the following minimum dimensions end up a little tight on the street but should still be viable. (In a perfect world, you would want about 0.100-inch more than shown from the piston deck to the first ring, as well as between each ring groove.) These dimensions assume the use of a conventional 1/16-1/16-3/16 three-ring package and the popular 0.990-inch-od big-block Chevy piston pin. The big-block Chevy pin is plenty strong but saves some room because it's smaller than a standard Ford 460 pin. Plus, it's easier to order custom pistons and rods for the more popular Chevy pin.

Distance, piston deck to ring 1

0.150

Thickness, ring 1

0.063

Distance, ring 1 to ring 2

0.150

Thickness, ring 2

0.063

Distance, ring 2 to ring 3

0.100

Thickness, ring 3 (oil ring)

0.190

Distance, ring 3 to top of pinhole

0.050

Half the piston-pin diameter

0.495

TOTAL

1.261

At 1.261 inches, the ring-package requirement is smaller than the 1.272 max CD dimension. You should have enough clearance for the rings without having to run the piston pin through the oil-ring groove, which requires special retainers and is not generally recommended for long-term street durability.

Third, you need to determine what piston dome or dish volume is required to produce the desired compression ratio. The previously calculated dimensions will yield a piston with a 0.011-inch piston (below) deck clearance at TDC (1.272 - 1.261 = 0.011). Piston deck clearance is one of the variables used for calculating compression ratios. You also need to know the head-gasket compressed volume (or compressed thickness and the gasket bore size), the combustion-chamber volume, and the piston dish or dome volume. Various Web-based calculators can give you exact figures (just Google "compression-ratio calculator"), but Hank says a fairly accurate approximation of what's needed in terms of total combustion-cavity volume (CV, expressed in cubic centimeters) in a V-8 engine to achieve a desired compression ratio (CR) can be found using:

4/4At BDC, the piston must clear the crank counterweight. To determine counterweight height at BDC, subtract from the rod length half the stroke, half the main journal diameter, half the wristpin diameter, the piston thickness below the wristpin hole, and a 0.100-inch clearance value. If the result is negative, the crank or piston must be ground for clearance.

The piston's valve reliefs must be deep enough to avoid valve contact with your cam selection. Deep valve reliefs are pretty standard with aftermarket, custom pistons, but your piston maker may want more detailed info on your final cam specs. Regardless, you should still trial-assemble the engine and check piston-to-valve clearance in 5-degree increments on the overlap between the exhaust and induction stroke, beginning at 20 degrees before TDC and ending at 20 degrees after TDC. At running lash, you want about 0.080/0.100 inch clearance on the intake and exhaust valves, respectively. In a worst-case scenario, the pistons must be sent out for fly-cutting.

At bottom dead center (BDC), the piston must clear the crank counterweight. Most pistons designed for stroker engines have ample clearance, but again, this should be verified during mockup. Generally, you want about 0.100 inch clearance between the piston and the crank counterweight. If necessary, either the crank counterweight or the piston can be remachined to achieve this. Do not remachine the counterweight in a circular arc or there will be insufficient weight left for balancing purposes. Instead, the counterweight must be cam-ground for clearance by an experienced, custom crank shop.

Lastly, a 12.5:1 static compression ratio almost certainly requires race gas and is way too high for a supercharger. You also really need good aftermarket aluminum heads to support such a large engine. For a cam, you should be looking for something over 0.600 lift with duration in the mid-250s or higher (at 0.050-inch tappet lift).